The present invention relates to cooling engines in general and, more particularly, to cooling engines having a free motion, gas-driven, piston actuated displacer.
Traditionally, free displacer, i.e., free piston cooling engines, work well thermodynamically, but lack sufficient reliability over a long period of time for them to be commercially successful against the currently available mechanical driven cooling engines. The problem with a free, gas-driven displacer is controlling the phase relationship between the position of the displacer and the pressure pulses of the working fluid at the top dead center and the bottom dead center of its cycle. In order to achieve high thermodynamic efficiency, the volumes at top dead center (TDC) and bottom dead center (BDC) should approach zero. With free displacer machines, this objective is very difficult to achieve without collisions taking place between the displacer and cylinder containing the displacer.
U.S. Pat. No. 4,792,346, issued Dec. 20, 1988, for a "Method and Apparatus for Snubbing the Movement of a Free, Gas-Driven Displacer in a Cooling Engine" discloses a method for snubbing displacer movement that utilizes a magnetic repulsion force between the displacer and each end of the cylinder containing the displacer. Two stationary magnets are placed at the ends of the displacer containing cylinder and the displacer itself has two movable magnets attached to the ends of the displacer in such a manner that they act as magnetic springs, i.e., the like magnetic poles of the stationary and movable magnets at one end face each other and, similarly, the like magnetic poles of the stationary and movable magnets at the other end of the displacer and cylinder face each other.
As the displacer approaches one end of the cylinder, the repulsion force of the magnetic force of the magnetic spring stores the kinetic energy of the displacer and prevents a collision from taking place. When the displacer is allowed to move in the other direction, the stored energy is converted back into kinetic energy in the opposite direction. Thus, the displacer is essentially suspended between the two magnetic repulsion forces which prevent collisions between the displacer and the ends of the displacer containing cylinder. The disclosure of U.S. Pat. No. 4,792,346 is incorporated herein by reference.
U.S. Pat. No. 3,991,586, issued Nov. 16, 1976, for "Solenoid Controlled Cold Head for a Cryogenic Cooler" discloses a closed cycle cryogenic cooler, utilizing two solenoids that selectively drive or selectively brake the regenerator-displacer. The physical position of the regenerator-displacer is used to control the actuation of the solenoids. The disclosure of U.S. Pat. No. 3,991,586 is incorporated herein by reference.
In order to achieve maximum cooling efficiency, the pressure/volume diagram ideally should be a perfect rectangle. Stated in terms of the displacer movement, the displacer should remain stationary while the pressure changes occur and commence its movement from TDC when a predetermined pressure is reached and should move to BDC without significantly overshooting the BDC position. Similarly, the displacer should be retained at the BDC position until a predetermined pressure differential is reached and then the displacer should move to TDC without significantly overshooting the TDC position.
It is accordingly a general object of the present invention to provide both a method and apparatus for controlling the phase relationship between the movement of a free piston, gas-driven displacer and the pressure pulses of the working fluid in cooling engine.
It is a specific object of the invention to utilize magnetic forces to provide the desired controlling action of the free piston, gas-driven displacer.
It is a further object of the invention to utilize both magnetic retaining forces and magnetic snubbing forces to provide the desired controlling action for the free, gas-driven displacer.
It is a feature of the invention that the method can be practiced and the apparatus constructed utilizing relatively inexpensive and commercially available magnetic components.